Located near Central Park on the Upper East Side of New York City’s Carnegie Hill District, the Carhart Mansion is a landmark building completed in 1916. It was built for Marion Carhart, the widow of Amory Carhart, a banking and railroad magnate. The mansion set the trend for subsequent mansions built in the French Neo-Classical style in the neighborhood. However, Marion Carhart died before she could occupy her home. Designed by the architect Horace Trumbauer (1868-1938) as a one-family residence, the five-story gilded Parisian-style townhouse had been used up until 2001 by Lycée Français as a French grammar school. Immediately adjacent and to the east of the mansion, a two-story nondescript building, built in 1957, was used by Lycée for classrooms and a gymnasium.

In 2001, the school sold both properties to a developer who planned to renovate the mansion, demolish the two-story structure, and erect a new building in its place, thus creating a single luxury condominium on the two sites. Our firm, Hage Engineering, was engaged to provide the structural design. The architectural firm Zivkovic Associates was hired to design the project. Zivkovic’s design proposed a six-story residential replacement building that would seamlessly connect to the mansion. The British architect John Simpson partnered with Zivkovic Associates and advised on the classical elements of the new façade. Sciame Construction Company was selected as the general contractor. The design was accepted and approved by the city and the Landmarks Commission and was deemed complimentary to the adjacent buildings and neighborhood.

The existing 32,000-square-foot mansion contained double-story floors — which were tall enough to allow for the insertion of new floors — that originally were used for ballrooms and dining rooms. A large marble curving staircase within the lobby was preserved, and a penthouse covering a portion of the roof was enlarged. The new building would add 17,000 square feet and boast a private garden in the rear, a new service elevator, a penthouse with an outdoor terrace, and a rooftop lap pool.

Mansion renovation
The structural system for the mansion consisted of steel beams and girders that bore on solid multi-wythe brick load-bearing walls around the perimeter, supported by steel and cast iron columns at the interior bays. The floors were constructed by formed cinder slabs (flat arches) that spanned between the steel beams, which are spaced roughly 5 to 6 feet-on-center. The cinder slabs were reinforced by draped wire mesh continuous over the top flanges of the beams and hooked at discontinuous ends. At the time, this type of construction was slowly supplanting the popular terra cotta arch systems for a dominant market share of fireproof floor construction in New York City.

To create new floor levels and new penetrations, steel framing was erected with new composite metal deck concrete slabs. Testing on the existing steel was performed to determine the grade and weldability of the steel, which was determined to be weldable. The design team was assisted by the original structural drawings and steel shop drawings that were buried within New York City’s archives and uncovered by the Office of Metropolitan History. An inordinate amount of floor probes would have been needed to determine the original building framing without the benefit of the original drawings.

Part of the mansion’s renovation included new steel framed floors with lightweight concrete topping over varying depths of composite metal decks. These slabs were inserted between existing floors to create duplexes and were used for the enlargement of the existing penthouse. Because of the additional lateral loads, the existing steel framing at the penthouse was analyzed and upgraded by reinforcing the existing connections and girders.

Since the façade of the building is landmarked, the team decided to address the deterioration in the masonry and to restore it to its original appearance. The walls consisted of multi-wythe brick with limestone cladding on the exterior. For the most part, the backup brick walls were in good condition with only minor repointing and rebricking required. The bulk of the work consisted of removing and resetting limestone pieces that were cracked or broken. Lastly, the building’s façade was power-cleaned to its original appearance.

Existing steel beam and girder connections to existing cast iron column were all tested for grade and weldability. A cast-in-place, full-height concrete wall proved to be the best solution for the new building’s front wall (shown here with concrete formwork in place during construction). the existing two-story building was demolished, but the existing foundation walls (on the right in the photo) remained as a temporary soil retention system.

New building
Geotechnical investigations at the site revealed a water table and poor soils close to the elevation of the new basement level. Soil borings also revealed bedrock at approximately 30 feet below that elevation. In order to limit dewatering and reach the required rock bearing level, the geotechnical engineers at Pillori Associates recommended a deep foundation system consisting of drilled, 120-ton, mini-caissons filled with high-strength grout. The caissons’ actual capacity was later value engineered by Sciame Construction to a higher capacity, which allowed for the use of one caisson below certain columns. The mini-caissons were socketed into the bedrock and braced with reinforced concrete pile caps and grade beams.

Since a large excavation was being undertaken, a soil retention system was contemplated in order to maintain the stability of the adjacent landmarked Fabbri Mansion and its plaza. Rather than installing a new soil sheeting and bracing system, the team decided to keep the existing foundation walls from the demolished, two-story gymnasium and use them as part of a temporary soil sheeting and shoring system. These walls were braced into the interior area of the excavation until new abutting foundation walls were built. The new foundation walls were placed inboard of the extant walls and were designed to retain the soil load on a permanent basis. Since the new foundation walls were set back from the new walls above, large concrete beams were designed at the first floor to cantilever over the old foundation walls and to support the new facades for the full height of the building.

Another challenge for the team was aligning the new floor plates with the existing elevations at the mansion. Based upon conceptual analysis of various floor assembly systems, a two-way reinforced concrete slab floor system was chosen. With a total assembly thickness of only 9 inches, this slab offered the most flexible system and allowed for ductwork and finishes to align with the existing floors and provide the needed head height for a New York City luxury apartment. A steel system was explored and rejected because the floor assembly was too deep.

The concrete slabs spanned between the concrete columns and concrete walls at the elevator core. In order to limit deep transfer beams, each floor layout was re-examined and modified to align columns at the same location on each floor. This allowed the contractor to use the same formwork at multiple levels. However, at the penthouse, where architectural layouts could not be adjusted, single-story height concrete columns were transferred onto columns below by composite concrete and steel beams. This maximized needed head height and minimized long-term deflection. At the new penthouse roof, concrete beams were incorporated to support a future rooftop lap pool.

The façade design for the three new exposed sides of the new building called out for stone veneer. Concrete spandrel beams with reinforced CMU backup walls were initially explored. After reviewing the window penetration geometry, engineers determined that only minimal areas of the wall would require CMU backup. In order to eliminate steel lintels and awkward CMU construction, the front and rear backup walls were constructed of reinforced concrete with punch-outs within the formwork to allow for windows.

These full-height concrete walls at the front and rear of the building were used as shear walls to resist the wind and seismic lateral forces. To resist the lateral forces in the perpendicular direction, the full height of concrete walls around the stair and elevator core were used as shear walls. The original building was built prior to the New York City’s seismic code enactment. Therefore, at the beginning of the design process, the design team decided that the new building would not be attached to the original building. Had the two buildings been connected, the original building’s unreinforced masonry walls and footings would have needed reinforcement. To accommodate this separation, a seismic joint was placed between the new and old buildings.

To maintain the schedule, the shear walls were poured prior to determining the location of some of the mechanical openings that were required at the stair walls. Once those openings were located, a new analysis was performed on the shear walls. To reinforce the new openings, new reinforcing steel plates were added at the perimeter of the openings, where needed.

Summary
The project received the Palladio Award for classical architecture in 2006. The restored mansion and the new building combined to create a seamless contextually respectful addition to the neighborhood. A thorough understanding of the archeology of turn-of-the-century structural systems and diligent initial research into as-built conditions allowed for selections of optimal systems. This focused approach at an early stage of determining the unique issues relating to a hybrid urban building allowed for sound conceptual selections that resulted in a construction phase that met the projected timelines.

Principal Mark Hage, P.E., Associate William Doll, P.E., and Senior Associate Richard Lee, P.E., S.E., LEED AP, are all with Hage Engineering, which was founded in 1990. They can be reached at 212-358-7778 or via e-mail at mhage@hageengineering.com, wdoll@hageengineering.com, and rlee@hageengineering.com, respectively.